50 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 



sufficient refrigeration to take care of the neat which finds its way 

 through the insulation of the walls, floors, and ceilings of the cold- 

 storage rooms. The greater the efficiency of the insulation the less 

 heat will get through from without. There is a limit, however, to the 

 amount of insulation that should be installed, which is the point 

 where the interest on the money invested in insulation, the repairs 

 and depreciation on same, balances the saving in operating expenses. 



There is no material known that will entirely prevent the passage 

 of heat. However, there are some which offer a very high resistance, 

 and are therefore termed nonconductors or insulators. The best 

 heat insulators appear to be those that contain the greatest amount 

 of entrapped air confined in the smallest possible air space. 



The function of cold-storage insulation, then, is to prevent the 

 outside heat from passing through the walls, floors, and ceiling into 

 the interior of the cold room. Therefore the problem is to minimize 

 the passage of heat by interposing in the walls, floors, and ceiling a 

 material or construction which will resist the transfer of heat from 

 the outer to the inner side of the room. The materials most commonly 

 used for this purpose are the different varieties of cork products, 

 mineral wool, hair felt, rock wool, vegetable fiber, sawdust, mill 

 shavings, etc., used in combination with wood, cement, masonry, and 

 air spaces. At one time it was common practice in the construction 

 of buildings for cold-storage purposes to provide a series of air spaces 

 in the walls, some of which were as much as 12 inches wide, the 

 supposition being that they were dead-air spaces. As a matter of 

 fact they were not. As the air in contact with the warmer surface 

 became heated it rose, while that in contact with the cooler surface 

 fell, thus producing a circulation tending to equalize the temperature 

 of the sides of the air space. Dead air, however, is a good nonconduc- 

 tor, but unless the air spaces are properly proportioned, the above- 

 mentioned air currents will be set up. Therefore, it is the present 

 practice to fill in the spaces with some porous substance to break up the 

 space into an indefinite number of small dead-air spaces which will 

 effectually prevent circulation of the entrapped air. On the other 

 hand, there is danger of packing the insulating material too closely, 

 which will result in favoring the conduction of heat through the 

 walls. 



Sawdust and mill shavings are mentioned in the above partial list 

 of insulating materials, but they are not to be considered among 

 the best. They can be had in any part of the country, and often 

 without cost, and if kept dry are good insulators. It is a very difficult 

 problem, however, to keep them dry, and when used, great care 

 should be exercised in the construction and workmanship of the walls 

 in order to keep out the moisture. 



